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Dolphin Pool & Spa

Dealing with Pool Stains That Appeared Over Winter

Thursday, May 7, 2026  ·  Your Happy Dolphin Pool & Spa Team

Pool staining on liner and walls after a Minnesota winter

Images are AI generated and do not depict an actual pool or Dolphin Pool & Spa location.

You pull back the cover, peer into the pool — and there they are. Brown streaks along the waterline. Rust-colored blotches on the floor. A greenish tinge on the walls that wasn't there in September. Winter staining is one of the most common surprises pool owners face at opening, and the good news is that most of it is treatable if you identify it correctly and attack it with the right products.

The bad news? Using the wrong treatment on the wrong type of stain won't just fail to work — it can actually make things worse. Here's how to identify what you're dealing with and what to do about it.

1 Identify the Stain Type First

Pool stains fall into two broad categories — organic stains and metal stains — and they require completely different treatments. Treating a metal stain with an organic stain remover, or vice versa, is a waste of time and money. Here's how to tell them apart:

Stain Type Typical Colors Common Causes
Organic Green, brown, black, yellowish Leaves, algae, debris, tannins from organic matter sitting on the liner over winter
Metal — Iron Rust, reddish-brown, orange Iron in source water, corroding metal fittings or accessories left in pool over winter
Metal — Copper Blue, green, teal Copper in source water, corroding heater heat exchanger, copper-based algaecides
Metal — Manganese Purple, black, dark brown Manganese in well water or municipal water, more common in certain MN areas

💡 Quick Field Test for Metal vs. Organic:

Take a small amount of granular shock and drop it directly onto the stain. If the stain lightens or disappears, it's organic — shock is working on it. If the stain darkens or turns darker brown/black, it's a metal stain — oxidizing it made it worse, which is your confirmation. Stop adding chlorine and switch to a metal treatment immediately.

2 How to Identify Algae Staining

Algae staining is one of the most misunderstood types of pool staining — mostly because by the time you open the pool in spring, the algae itself may be long dead, leaving only the stain behind. Here's how to tell if what you're looking at is algae-related:

  • Green or yellowish-green staining on walls and floor — classic green algae that bloomed and died under the cover
  • Black spots with a slightly raised or rough texture — black algae, which has roots that penetrate into plaster and grout and is the hardest to remove
  • Yellow or mustard-colored powdery deposits in shaded areas of the pool — mustard algae, which brushes away easily but keeps coming back
  • Staining that follows the waterline or appears in consistent patterns on walls — often algae that grew along a surface rather than floating in the water
  • Brush test — brush the stain firmly. If it partially dislodges or smears, there's likely still living or recently dead algae involved. If it doesn't budge at all, it may have already set as a permanent stain.

⚠️ Important: If you have active algae — not just staining but cloudy water, slippery walls, or visible growth — deal with the algae first before treating for stains. Shocking and brushing to eliminate the algae is always step one. Stain treatment comes after the water is clear and chemistry is balanced.

3 Treating Organic Stains

Organic stains respond to oxidation. Once your water is balanced and your chlorine levels are in range, a good shock treatment will often fade or eliminate light organic staining on its own. For more stubborn organic stains — tannin staining from leaves that sat on the liner all winter, for example — make sure your chlorine is up, brush the affected areas, and give it time to work. Natural Chemistry Stain Free is also effective on many organic stains. Bring in a water sample and we'll help you build a treatment plan.

💡 Pro Tip: The faster you get your water chemistry balanced and chlorine working after opening, the less time organic stains have to set. Fresh organic stains are far easier to treat than ones that have been sitting in water for weeks. Don't delay your opening chemistry — it pays off in a cleaner liner.

4 Treating Metal Stains

Metal stains require a different approach entirely. Chlorine and shock make metal stains worse — oxidation causes dissolved metals to plate out onto pool surfaces more aggressively. If you have metal staining, you need to lower your chlorine before treating, and hold off on shocking for at least 4 days after treatment. We carry three products specifically for metal staining:

Product Best For How It Works
BioGuard Erase Existing iron stains on pool surfaces — also removes cobalt staining on fiberglass Granular formula added directly to pool water over the stained area. Chlorine must be below 1 ppm before use. No shock for 4 days after treatment. Follow immediately with Pool Magnet Plus to prevent re-staining.
BioGuard Pool Magnet Plus Iron and copper staining prevention — used after Erase treatment and as ongoing weekly maintenance Sequestering agent that binds metals and keeps them suspended in solution so the filter can remove them. Essential follow-up after Erase and ongoing treatment for pools with metal in the source water.
Natural Chemistry Stain Free Iron and copper metal stains on pool surfaces Citric acid-based formula that lifts existing metal stains off surfaces. Works on iron and copper staining. A good option when stains have already set and you need a thorough surface treatment.

💡 Metal Stain Strategy — Think in Two Steps:

Step 1 — Remove: Use BioGuard Erase (iron) or Natural Chemistry Stain Free (iron and copper) to lift existing stains off the surface. Step 2 — Prevent: Follow up immediately with BioGuard Pool Magnet Plus and maintain a weekly dose throughout the season to keep metals sequestered. If you have metal in your source water — and many Twin Cities homes do — skipping the preventative step means the stains will be back by fall.

5 Get Your Water Tested First

Before you treat for anything, bring a water sample in to Dolphin Pool & Spa for a free water test. A full water analysis will tell you whether you have elevated metals in your water — which changes your entire treatment approach — and will make sure your chemistry is balanced before you add any stain treatment products. Adding stain removers to water that is out of balance is far less effective and can cause secondary problems.

✅ Free water testing with $50 in chemical purchases. Stop in with a sample from elbow-depth in your pool and we'll run a full analysis and build you a treatment plan. We carry BioGuard Pool Magnet Plus, Natural Chemistry Stain Free, BioGuard Erase, and everything else you'll need to get your pool looking great for the season.

Not Sure What You're Looking At?

Bring In a Water Sample — We'll Figure It Out.

Stain identification and treatment is one of the trickiest parts of pool ownership. We've been doing this since 1988 — bring in your water sample and a photo of the staining and we'll point you in the right direction. We're here Mon–Thurs 8–5, Fri 8–6, and Sat 8–4.

Call Us Now — 763-542-9000
Dolphin Pool & Spa

Your Happy Dolphin Pool & Spa Team

3405 Hwy 169 N, Plymouth MN  ·  763-542-9000  ·  dolphinpool-spa.com